1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for attaching a composite shroud to plural blades of an impeller.
2. Description of the Prior Art
Compressors are a particular type of turbo-machine that is able to increase a pressure of a compressible fluid (e.g., gas) by using mechanical energy. Various types of compressors are used in processing plants in the oil and gas industry. Among these compressors, there is the so-called centrifugal compressor in which energy is supplied to the gas particles by centrifugal acceleration. The centrifugal acceleration may be achieved by the rotation of a rotating member composed of one or more wheels or centrifugal rotors made of metal, and housed inside a stator.
A centrifugal compressor may be fitted with a single rotor (impeller), the single stage configuration, or with several rotors arranged in series, the multi-stage compressors. Each stage of a centrifugal compressor includes a suction duct in the stator for the gas to be received, a rotor which is able to supply the kinetic energy to the gas, and a ducting system within the stator, between one rotor and the following rotor for the purpose of converting the kinetic energy of the gas leaving the rotor into pressure.
Another type of turbo-machine is the pump, which is able to increase the pressure of a non-compressible fluid (e.g., liquid) by using mechanical energy. Various types of pumps used in the oil and gas industry include the so-called centrifugal pump, in which mechanical energy is supplied to the liquid in the form of centrifugal acceleration, by rotating a rotating member composed of one or more wheels or centrifugal rotors, and generally made of metal in the case of high performance turbo-machines. Centrifugal pumps may be fitted with a single rotor or a number of rotors arranged in series and housed within a stator. The centrifugal pumps may be formed with suitable expansion and return ducts in order to convert the kinetic energy of the liquid leaving the rotor into pressure.
Centrifugal rotors, whether for centrifugal compressors or centrifugal pumps, are generally classified as rotors with “single”, “two-dimensional” or “three-dimensional” curvatures depending on their geometry. In particular, a 2D centrifugal rotor in two dimensions (see FIG. 1A) includes blades 302D possessing a profile that substantially extends in two dimensions around a rotational axis X2D of the rotor. In general, blades 302D have a profile that is radial from an inlet orifice 200I towards an outlet orifice 200U. In addition, the 2D rotor includes a backing disc 202D, which is radial with respect to the axis of rotation X2D and to which the blades 302D are attached.
FIG. 1B shows a two-dimensional rotor 22D similar to that shown in FIG. 1A, but which differs from the former in that it possesses a front disc or backing disc 102D, fixed onto the side of the blades 302D opposite to the backing disc 202D as discussed later in more details.
FIG. 1C shows an enlarged cross section of the rotor 22D, in which the backing disc 102D is shown fixed to blades 302D, and which presents a substantially radial development, suitable for forming throughput spaces for the fluid, and at the same time, guiding it from the inlet orifice 200I to the outlet orifice 200U. In particular, in the two-dimensional rotor 22D, the backing disc 102D has a “plate-like” shape with a surface S1 which is substantially parallel to the axis X2D, which is shaped in the vicinity of the inlet orifice 200I in order to facilitate the inlet of the fluid, and a plane and radial surface S2, i.e., a normal surface joined to the surface S1 so as to extend from the inlet orifice 200I to the outlet orifice 200U.
A 3D centrifugal rotor in three-dimensions is shown in FIG. 2A and the 3D rotor is characterized by blades 303D possessing a substantially three-dimensional profile around a rotational axis X3D of the rotor from the axial inlet orifice 303I towards a substantially radial outlet orifice 303U. In other words, this type of blade 303D is not generated from a simple axial translation with an aerodynamic profile, as is the case for the two-dimensional blades, but can be produced with any profile suitable for maximizing the fluid dynamic performance of the component. This type of 3D rotor has a backing disc 203D to which blades 303D are attached.
FIG. 2B shows a rotor 33D in three-dimensions similar to that in FIG. 2A, but which differs from the former due to the fact that it includes a front disc or backing disc 103D fixed onto the side of the blades 303D opposite to the backing disc 203D. FIG. 2C shows a cross section of the rotor 33D in which the backing disc 103D is fixed to the blades 303D. The backing disc 103D has a profile, either axial or radial, suitable for shaping the throughput spaces for the fluid, and at the same time for guiding it from the inlet orifice 300I to the outlet orifice 300U. In particular, in the case of three-dimensional rotors, the backing disc 103D generally has a substantially “bell-shaped” or “trumpet-shaped” profile, in which the direction normal to its external surface S3 passes gradually from an approximately radial to an approximately axial direction from the inlet orifice 300I to the outlet orifice 300U. However, it should be noted that the form of the backing disc 102D and 103D may vary depending on the particular application.
It is also possible to have configurations possessing characteristics intermediate between a two-dimensional and a three-dimensional rotor depending on the particular application, such as for example rotors possessing two-dimensional blades, and a backing disc with a substantially bell-shaped form, or other configurations.
A centrifugal rotor, either of a two-dimensional type (FIGS. 1B and 1C) or of a three-dimensional type (FIGS. 2B and 2C), is traditionally called “closed” if it possesses a backing disc 102D or 103D. On the other hand, the rotor is commonly called “open” if it does not possess such a backing disc 103D or 203D. In the latter case, it is the stator casing of the turbo-machine that guides the process fluid being propelled within the rotor. The above-mentioned rotors can be produced by assembling their components (by means of welding or brazing), or from a single solid metallic body by mechanical machining (for example by machining or electro-erosion), casting or other means. Each of the above-mentioned types of centrifugal rotor present specific advantages and disadvantages, some of which are summarized below.
Two-dimensional centrifugal rotors, whether open or closed, are simpler and more economical efficient than open or closed three-dimensional rotors due to their geometry. In turn, open centrifugal rotors, whether two or three-dimensional, are simpler and more economical efficient to be produced compared to two or three-dimensional closed rotors because they do not possess a shroud, which traditionally complicates the work necessary to produce them.
On the other hand, rotors of the two or three-dimensional closed type, achieve a better flow control compared with open two or three-dimensional rotors as they possess a well-defined fluid dynamic pathway. Moreover, closed type rotors permit the achievement of a higher performance as they minimize flow losses in relation to the stator casing.
A disadvantage of open or closed two-dimensional centrifugal rotors is the fact that they possess a fluid dynamic performance which is inferior to open or closed three-dimensional centrifugal rotors, because of their geometry.
Another disadvantage of the two or three-dimensional open centrifugal rotors is the fact that they possess a fluid dynamic performance inferior to that for two or three-dimensional closed rotors because of the leakage of fluid between the rotor and the stator housing, which is particularly relevant in multi-stage turbo-machines, in which it is difficult to keep an axial deformation under control.
A disadvantage of the two or three-dimensional closed-type rotors is the fact that they have a maximum peripheral velocity that is lower than that of the two or three-dimensional open type. This is due in particular to the fact that the backing disc creates a centrifugal tension (indicated by Fc in FIGS. 1C and 2C) on the blades due to its radial expansion, which is particularly relevant at high rotational velocities. In particular, above a certain velocity (generally around 350 meters per second, depending on the material and the geometry), the backing disc generates such a high tension in the blades that it can lead to the destruction of the rotor itself.
Thus, two-dimensional open rotors have proved to be the most simple and economical to manufacture, have a high maximum rotational velocity, but at the same time have a low fluid dynamic performance. On the other hand, three-dimensional closed rotors prove to possess the highest fluid dynamic performance, but at the same time are the most complex and expensive to manufacture, and have a limited maximum rotation velocity. Rotors of intermediate type, e.g., two-dimensional closed rotors or three-dimensional open rotors, possess intermediate advantages.
U.S. Pat. No. 4,676,722, the entire content of which is incorporated herein by reference, describes a centrifugal rotor that possesses increased mechanical strength and reduced weight in order to obtain a rotational velocity and a diameter of the rotor that are greater compared with those of traditional rotors. This rotor is produced with a series of scoops formed of a composite material and rigidly fixed to each other in a circumferential direction with respect to its axis of rotation.
One disadvantage of the above-mentioned rotor is the fact that the various scoops comprise reinforcement fibers substantially oriented in a radial direction, which means that it is difficult to balance the circumferential tension due to a centrifugal force Fa (see FIG. 1C) that arises at a high rotational velocity.
A further disadvantage of the rotor of this document is the fact that such a rotor is relatively complex from a mechanical point of view, since it is composed of many different components that must be produced independently and mechanically assembled together. Moreover, this mechanical assembly cannot easily be achieved using automated machines which results in increased manufacturing times and costs.
Another disadvantage of this rotor is the fact that the composite material of the rotor in the region of contact with the flow is not protected from the wear caused by the possible presence of solid particles in the flow, or from possible acid fluids.
Still another disadvantage is the fact that it may prove difficult to achieve the tolerances for each component and for the fixing systems that are necessary for optimum functioning of the rotor at high velocity. Moreover, possible deformation produced by the tensions and forces created during use may lead to problems in operation. In addition, a vibration may occur during operation, caused by wear and/or by imperfect mechanical assembly of the various components.
Patent JP 56132499, the entire content of which is incorporated herein by reference, describes a closed centrifugal rotor which possesses a ring formed of a composite material and arranged at the intake side of the backing disc to attempt to reduce the centrifugal tension produced at high rotational velocity. One disadvantage of this rotor is the fact that such a ring formed of a composite material acts in an extremely localized manner in the region in which it is installed, for which reason its reliability is not particularly high.
Another disadvantage is that appreciable shear forces are generated between the ring and the backing disc at high rotation velocities, due to deformation of the backing disc, so that dangerous cracks can be formed in the backing disc.
Still another disadvantage is the fact that a deformation of the backing disc increases with a rotational velocity, resulting in a risk that the ring may become detached. If this happens, the rotor would disintegrate, thus damaging the parts of the stator of the machine.
Patent JP 9195987, the entire content of which is incorporated herein by reference, describes a centrifugal rotor of the “three-dimensional” type. On the backing disc is bonded a layer of composite material including carbon fiber to increase its rigidity and limit a deformation in the region of the outlet orifice at high rotation velocities. One disadvantage of this rotor is that the rotor does not solve the problem of the centrifugal tension Fc that is generated in the backing disc at high velocities. The increase in velocity obtained through this arrangement is therefore limited for a closed rotor.
Another disadvantage is that, under conditions of maximum velocity, the layer of composite material may become detached from the plate, since the load acts tangentially to the backing disc, generating high shear forces in the bonding surface.
Patent JP 141898, the entire content of which is incorporated herein by reference, describes an open centrifugal rotor which possesses a backing disc with a coaxial cavity to reduce its weight, and an annular element arranged on the peripheral surface of the backing disc in order to reduce its deformation. This annular element is produced of a material having a coefficient of thermal expansion that is lower than that of the rotor.
One disadvantage of this rotor is that, even in this case, this system does not solve the problem due to the centrifugal tension Fc that is generated in the backing disc at high velocities. The increase in velocity obtainable by this arrangement is therefore limited for a closed rotor.
In conclusion, none of the above documents solve the problem of the tensions generated by the backing disc in the blades at high rotational velocities. Therefore, in spite of developments in technology, it is still considered necessary to produce centrifugal rotors of the “closed” type for turbo-machines that can operate at higher rotational velocities, while at the same time guaranteeing sufficient reliability and economy in production and use.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.